Diesel vs petrol cars

Choosing between petrol and diesel can be tricky. From cost and performance to environmental impact, we’ll break down the facts to help you decide which is best for your car.

What's the difference between petrol and diesel cars?

There are a number of differences between petrol and diesel cars. Driving experience, cost, and the environment are all things to consider when working out what's best for you.

What's cheaper diesel or petrol cars?

Petrol cars are generally cheaper to buy, and petrol is usually cheaper per litre than diesel. Diesel is more economical with fuel use though, so the difference in running costs is usually fairly small. Check our petrol prices guide to find the price of petrol and diesel in your local area.

One of the main reasons that diesel cars are more expensive is the engine. Diesel cars ignite fuel by compressing it to a very high pressure rather than using a spark plug. A spark plug is a small part of a car's ignition system. It ignites the air and fuel mixture in the engine's combustion chamber, creating the power that makes the car move.

This means that the engine has to be stronger in a diesel car. There's also the extra cost of emission-control systems like diesel particulate systems and AdBlue. This is a diesel exhaust fluid that helps reduce nitrogen oxide emissions from diesel engines.

Running costs

There are a few factors that affect running costs. Let's go through each one and see how diesel and petrol compare:

• Insurance. Petrol and diesel are on a roughly even playing field when it comes to car insurance. A diesel car may have slightly higher insurance rates because of potentially high repair costs. Overall though, the difference is slim.
• Servicing. Because diesel cars have more systems, there's more things that could go wrong. Older diesel cars tend to have worse MOT pass rates than petrol cars, so this could lead to more expense.
• Fuel. Diesel tends to be more expensive per litre than unleaded petrol, but diesel is usually more economical with fuel use.
• Tax. Diesel cars generally emit less CO2 than petrol cars, but the difference is small. This can affect how much tax you pay. The current tax system works out your first-year rate by the amount of CO2 it emits. ULEZ (Ultra-Low Emission Zone) have also made it harder for diesel cars. Diesel cars need to meet Euro 6 standards, and petrol cars Euro 4. Diesels registered from around September 2015 generally meet Euro 6, but compliance depends on the car’s declared emissions, so use TfL’s vehicle checker to be sure.
• Emissions. Older diesel cars can be polluting, and can emit more air pollutants like nitrogen dioxide (NO2). AdBlue and diesel particulate filters have helped to cut this down though. Diesel cars generally emit less CO2 than petrol, but the margins are small. Diesel engines are more efficient with fuel consumption, so they use less fuel to travel the same distance.
• AdBlue. This is diesel exhaust fluid (DEF) that helps to reduce harmful emissions. You'll need to refill this tank periodically. The cost depends on how and where you choose to fill it, but currently it would cost around £10-£20 to fill up a 10 litre AdBlue tank. How often you fill your car with AdBlue can depend on the size of your tank, your driving style, and the number of miles you drive. A 10-litre bottle will usually be enough to get you at least 3,000 miles. 

Purchase price

Diesel cars are typically more expensive to buy than petrol cars. This isn't always the case though, and the price can depend on the age, make, and model of the car.

For example, a used diesel car can be cheaper than a new petrol car.

Car value

Many factors can affect the value of your car in the UK market.

Petrol:

Petrol cars are extremely popular in the UK. They're cheaper to buy than diesel cars and can be more economic for shorter journeys and city driving. Their values could be affected as more people move to electric and hybrid cars, and by low emission zones such as ULEZ and Clean Air Zones.

Diesel:

The same may apply to diesel cars as the UK moves towards ending new petrol and diesel car sales from 2030.

Diesel cars use less fuel than petrol cars of a similar size, which can help running costs on long journeys. How well a diesel car holds its value depends on its age, model, mileage and emissions standard, and it could be affected by low emission zones and future rules. Don’t assume any car will hold its value, and check what similar cars are selling for before you buy.

Driving and Performance

There are a few differences between how petrol and diesel cars drive. Let's take a look through them:

Noise

Diesel cars are typically noisier than petrol cars. Though diesel cars have become more refined in recent years, the engines are usually louder than a petrol car.

Revs/engine speed

Petrol cars have a higher rev limit (how fast the engine can spin) than diesel cars.

Power delivery/torque

Diesel cars typically produce less peak power than petrol cars. Torque is the measurement of a car engine's ability to generate rotational force and turn the wheels. It's what gives drivers the feel of power and control in a car. Diesel cars typically produce more torque than their petrol counterparts. This makes them great motorway cars, and more suited to towing caravans, for example. They don't need to work as hard to accelerate, especially at high speeds. These factors also explain why diesel cars use less fuel, as they're not having to work as hard to gain speed.

Diesel particulate filters

While diesel cars are great for high-mileage drivers, they can prove less effective for low-mileage drivers. This is due to diesel particulate filters (DPFs). DPFs absorb tiny soot particles from a diesel's exhaust, trapping it in a filter. The trapped soot needs to be burned off so that the filter doesn't get blocked. This can only be done when the engine is up to operating temperature, and the car's engine and speed is above a certain level. So if you're someone who mainly drives in town and don't usually go above 50mph, you may want to consider a petrol or electric car instead.

What's the environmental impact of petrol and diesel cars?

The UK government has confirmed that the ban on new pure petrol and diesel cars will happen in 2030.

After 2030, dealers won’t be able to sell new pure petrol and diesel cars. They’ll only sell new zero-emission cars and hybrids. The government has set out a separate, later deadline: by 2035, all new cars, including full hybrids and plug-in hybrids, must be fully zero-emission.

At the moment, this legislation will only apply to new petrol and diesel cars. There's no indication that second-hand diesel and petrol cars will face any restrictions to their sale.

It's worth noting that the government could bring in fresh emission control standards before 2030, for example, they might bring in new low emission zones. At the moment though, almost all current zones work on a basis of legislating against pre-Euro 4 petrol cars, and pre-Euro 6 diesels.

But 2030 is still a long way off, so let's look at the environmental impact of both petrol and diesel.

Petrol

Petrol cars can have a significant impact on the environment. They emit carbon dioxide (CO2) and other greenhouse gases (GHGs). CO2 traps heat in the atmosphere which leads to global warming. They also produce methane and nitrous oxide, which are even more effective at trapping heat in the atmosphere than CO2.

The production and distribution of petrol cars also has a negative impact on the environment. Oil has to be extracted from the ground, transported to a refinery, and refined into petrol. This creates harmful GHGs before it's even transported to our service stations.

Exhaust fumes from petrol cars can add to air pollution, which can be harmful to health, particularly for people with existing lung or heart conditions. You can read more about air pollution and public health on the Defra website.

Diesel

Diesel cars also emit harmful pollutants, including nitrogen oxides (NOx), sulfur dioxide (SO2), and particulate matter (PM). These emissions can lead to smog and acid rain which damage crops, trees, and other vegetation.

Just like petrol, diesel cars also contribute to global warming by producing greenhouse gases. They do generally have lower CO2 emissions than their petrol counterparts though.

The emissions from diesel cars can still be harmful to health, particularly for people with existing lung or heart conditions.

What are the pros and cons of petrol cars?

Let's recap the main advantages and disadvantages of petrol cars:

Advantages

• Petrol cars are usually cheaper to buy.
• Their engines are typically more refined.
• Fuel is usually cheaper per litre.
• Potentially higher MOT pass rates and lower servicing costs.
• Generally fewer NOx and particulate emissions than older diesel cars.
• Most have no DPF to clog. It’s worth noting that some petrol cars now have particulate filters.

Disadvantages

• Higher fuel consumption. Petrol cars are less economical with fuel than diesel cars.
• Generally slightly higher CO2 emissions than diesel cars.
• Not so well suited to towing.

What are the pros and cons of diesel cars?

And on the other side, here's the main advantages and disadvantages of diesel cars:

Advantages

• Better fuel efficiency than petrol cars.
• Good motorway performance.
• Generally lower CO2 emissions.
• The engines don't have to work as hard.
• Longer range between refuelling stops.
• Well suited to towing.

Disadvantages

• They're more expensive to buy.
• Potentially higher servicing costs and lower MOT pass rates.
• The DPFs can get clogged.
• Loud engine noise.
• Older diesel models probably don't meet the emission standards for low emission zones.

Are electric and hybrid cars the future?

Now this is where it gets tricky. For many people, the shift towards electric and hybrid cars can be worrying. We all have different needs when it comes to buying a car. And we want to know that those needs can be met. Whether that's our budget, annual mileage, or our typical daily journeys. Let's take a look at what electric and hybrid cars can offer:

• Emissions: Electric cars (EVs) produce no exhaust emissions when you’re driving, which can help improve air quality where you live. Making the car and its battery, and generating the electricity to charge it, still have an environmental impact. Hybrids can produce lower exhaust emissions than petrol or diesel cars, depending on the model and how you drive.
• Fuel economy: Hybrid cars can use less fuel than petrol or diesel cars, especially around town, where regular slowing and accelerating lets the battery recharge under braking. Diesel can still suit long motorway journeys. Results vary by model and how you drive.
• Cost: Full hybrids (self-charging hybrids) typically cost less to buy than plug-in hybrids, though prices vary by model.

One of the main concerns for electric cars is charging access, especially on longer trips. There's also the issue of how electric cars are made. Making lithium batteries also uses large amounts of energy.

Over their lifetime, EVs in the UK typically produce less carbon than petrol or diesel cars, even after allowing for making the battery. The difference depends on how the electricity is generated, the size of the car and how long you keep it.

With new pure petrol and diesel sales ending in 2030 and all new cars zero-emission by 2035, EV sales are set to rise. Before deciding whether to buy an EV, it’s important to consider many factors, such as your driving habits, charging access, and overall cost of ownership. 

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